Every new generation of computer memory design provides increased information storage density. The typical structure size for a single binary storage cell has become quite small, on the order of about 0.5 μm per single cell, for instance. The physical limit to continued reduction in size of the structures which form electronics-based memory cells is rapidly being approached, however, and alternative methods for increasing the density of information stored in memory devices are being sought.
One method being developed to increase memory storage density is the construction of multi-bit memory cells (See, for example, U.S. Pat. No. 5,623,440 to Saito). The multi-bit memory cells of Saito are generally of the same basic construction as single-bit memory cells, consisting of a storage capacitor and a cell gate. The primary difference between the two is that the multi-bit memory cell can be used to store a plurality of bits through division of the voltage range of the cell into recognizable sub-ranges. For instance, in an electronic memory cell that is capable of storing voltage between 0 and 4 volts, the voltage range may be divided into four recognizable sub-ranges, and the memory cell can be a two-bit cell, rather than a single bit cell. In this manner, n bits may be stored in a single memory cell by creating 2n voltage sub-ranges.
Problems have been encountered in the development of multi-bit memory cells, however. For instance, electronic noise has been an on-going developmental problem. The more sub-ranges created in a memory cell, the smaller the voltage separation between sub-ranges, and a relatively small amount of electronic noise can alter the data. Another problem encountered in electronic multi-bit memory cell development has been that of limitation of the voltage range possible in these extremely small circuits. For instance, development of an actual dynamic range of a memory cell beyond about 3.5 volts has proven very difficult. Additionally, physical size constraints of the cells are still a problem, for even when a multi-bit cell can be developed, it is still limited in size by the physical limitations of the electronic structures which must be created on the chip surface.
Another method currently being developed for increasing the information storage density of memory devices includes using the tip of an atomic force microscope (AFM) for reading and writing topographic features on a substrate surface. In these devices, the data can be written on a substrate via thermomechanical processes wherein the surface of a substrate is deformed using nanolithography processes and a value is assigned to a memory cell based on the presence or absence of deformity in the cell. (See, for example U.S. Pat. No. 6,249,747 to Binnig, et al.). These methods, however, are limited to binary systems, and the width of an individual memory cell will correspond at least to the diameter of the AFM tip.
Nature has provided the premier information storage system in terms of both efficiency and effectiveness in DNA (deoxyribose nucleic acid) molecules, which are the basis of the genetic system of living organisms. For instance, a DNA molecule only 100 monomer units in length, using only four different nucleotides, can encode 4100 bits of information in a linear distance of about 100 nanometers. Moreover, these 4100 molecular bits, when combined together, may encode more than 1050 gigabytes of information.
While attempts have been made to artificially replicate this system, the problems encountered in the attempts have been many. For instance, while natural enzymes may recognize an individual monomer unit of a DNA strand, no artificial system developed to date can mimic this level of sensitivity. In addition, construction of an individual strand of DNA is only possible through very complex and time consuming enzymatic or chemical means. Even in nature, DNA polymer construction is not spontaneous; only with another DNA template to use as a guide can new DNA be constructed. Thus, even in nature, information in DNA may only be copied, and not created.
As such, what is needed in the art is a memory device which can include extremely small individual memory cells, for instance memory cells on the order of nanometers rather that micrometers. In addition, what is needed in the art is a memory device which is not limited to a binary-type information system. What is needed in the art is a computer system including a memory device which can mimic the storage density capability of DNA and DNA-like polynucleotides.